Lifting devices, such as bridge cranes and harbour cranes are used to move loads of extremely heavy masses weighing often tens or hundreds or even thousands of tons, for example. Because of the size of the masses to be moved, lifting devices must be operated according to strict safety regulations. The aim of safety regulations is to protect both the environment of the lifting devices, such as people and structures, and the lifting device itself.
The operating environment of the lifting device may be a warehouse, for example, in which the lifting device knows how to fetch loads and convey them between specified locations in the warehouse. The routes for the lifting device in question may be determined in advance, safety risks involved in the use of the lifting device thus being restricted to the routes it uses in the warehouse. The routes of the lifting device may be marked on the warehouse floor, for example, to allow the staff moving in the warehouse to keep away from the route and thus at a distance required by the safety regulations from the lifting device or its load. Since a specific space is reserved for moving the lifting device, other traffic in the warehouse requires a separate space reserved for it, the space being thus not available for storage of goods, which impairs the efficiency of surface space use in the warehouse. Since the route of the lifting device and/or the space reserved for it is often defined on the basis of the maximum amount of transferable load, the space to be reserved for the lifting device operation may be quite large in proportion to the size of the warehouse.
Nevertheless, a lifting device may generally be driven, contrary to the above policy, outside the marked route either accidentally or intentionally, and thus dangerous situations may arise despite the large space reserved for the operation of the lifting device.
When a lifting device is taken in use, a safety distance to fixed constructions of the warehouse, such as the walls and/or pillars, may be determined either fixedly or on the basis of a load weight measurement to provide a limit beyond which the lifting device cannot be driven. When the lifting device has been taken in use, the distance to the fixed constructions may be measured by a positioning system of the lifting device, the system being often based on laser measurement devices and/or on absolute or increment coders, which allow the movement of the load to be stopped at the safety distance from fixed obstacles. However, safety distances to fixed constructions do not take into account changes taking place in the warehouse that affect the movement of the lifting device. These may include other traffic in the warehouse, such as machines or people, and changes taking place in the warehouse, such as new walls or removal of walls.
Since the lifting device is unable to move within the determined safety distances after the device has been taken in use, the safety distances contribute to restricting the efficient use of the warehouse premises. Too large safety distances diminish the efficiency of use of the warehouse premises, whereas too narrow safety distances are not allowed due to security considerations.
In a situation in which the warehouse has been renovated by removing walls, for example, areas with no determined safety zones and/or routes have become part of the operating environment of the lifting device. In that case it is not possible to drive the lifting device safely in the renovated warehouse without re-introduction into use of the lifting device to determine the safety distances and/or routes. Introduction into use always requires that an expert of the lifting device supplier or serviceman is called to the site, which naturally causes costs to the owner and/or user of the lifting device.
New specifications for the lifting device may require a person specifically trained for the task, which may prevent the lifting device from being used until such a person has arrived. In other words, before new specifications are defined, the lifting device cannot be used for its ordinary tasks. However, in terms of economy, the downtime of an expensive apparatus like a lifting device should be kept as rare and short as possible. It is therefore desirable that downtime in the operation of the lifting device is as short as possible.
Moreover, a typical dangerous situation in crane use is caused by people and other machines, such as forklifts and trucks, moving in the vicinity of the load to be transferred. The crane operator does not necessarily always see people and/or machines coming in front of the machine behind a load. In addition, it is not possible to stop the load in an instant, and a sudden braking would swing a moving load to the direction of movement of the crane, possibly causing the crane to hit the object that has come in front of it. The swinging may be prevented by controlled braking, using the swing prevention mode of the crane. However, a controlled braking requires that the braking begins sufficiently early before the load is close to the object.